Biogenic amine O- and N-methyltransferases -catechol-O-methyltransferase (COMT), histamine-N-methyltransferase HNMT, phenethanolamine-N-methyltransferase (PNMT), hydroxyindole-O-methyltransferase (HIOMT), and indoleamine-N-methyltranferase (INMT) - have been shown to exist in mammalian tissues as multiple molecular forms. However, in no case has the nature of the enzyme multiplicities been known. The proposed work aims at purification and characterization of these enzymes and their multiple forms to determine their physiological functions in the metabolism and biosynthesis of biogenic amine hormones and neurotransmitters. Each individual enzyme will be extensively purified and studied with respect to its molecular structure, physicochemical, immunological, catalytic, and kinetic properties. Among these enzymes, COMT was previously shown by us to exist in human and rat liver as two distinct molecular forms, designated COMT I and COMT II, which differ in molecular size and charge. The two isozymes that have been purified from the latter tissue to electrophoretic homogeneity by affinity chromatography will be further characterized to determined the molecular basis for the enzyme multiplicity.